reloc(5) — RISC
Name
reloc − relocation information for a MIPS object file
Syntax
#include <reloc.h>
Description
Object files have one relocation entry for each relocatable reference in the text or data. If relocation information is present, it will be in the following format:
struct reloc
{
long r_vaddr ; /* (virtual) address of reference */
long r_symndx ; /* index into symbol table */
ushort r_type ; /* relocation type */
unsigned r_symndx:24, /* index into symbol table */
r_reserved:3,
r_type:4, /* relocation type */
r_extern:1; /* if 1 symndx is an index into the
external symbol table, else symndx
is a section # */
} ;
/* Relocation types */
#define R_ABS 0
#define R_REFHALF 1
#define R_REFWORD 2
#define R_JMPADDR 3
#define R_REFHI 4
#define R_REFLO 5
#define R_GPREL 6
#define R_LITERAL 7
/* Section numbers */
#define R_SN_NULL 0
#define R_SN_TEXT 1
#define R_SN_RDATA 2
#define R_SN_DATA 3
#define R_SN_SDATA 4
#define R_SN_SBSS 5
#define R_SN_BSS 6
#define R_SN_INIT 7
#define R_SN_LIT8 8
#define R_SN_LIT4 9
The link editor reads each input section and performs relocation. The relocation entries direct how references found within the input section are treated.
If r_extern is zero, it is a local relocation entry and then r_symndex is a section number (R_SN_*). For these entries, the starting address for the section referenced by the section number is used in place of an external symbol table entry’s value. The assembler and loader always use local relocation entries if the item to be relocated is defined in the object file.
For every external relocation (except R_ABS) a signed constant is added to the symbol’s virtual address that the relocation entry refers to. This constant is assembled at the address being relocated.
R_ABS The reference is absolute and no relocation is necessary. The entry will be ignored.
R_REFHALF A 16-bit reference to the symbol’s virtual address.
R_REFWORD A 32-bit reference to the symbol’s virtual address.
R_JMPADDR A 26-bit jump instruction reference to the symbol’s virtual address.
R_REFHI A reference to the high 16 bits of the symbol’s virtual address. The next relocation entry must be the corresponding R_REFLO entry, so the proper value of the constant to be added to the symbol’s virtual address can be reconstructed.
R_REFLO A reference to low 16 bits to the symbol’s virtual address.
R_GPREL A 16-bit offset to the symbol’s virtual address from the global pointer register.
R_LITERAL A 16-bit offset to the literal’s virtual address from the global pointer register.
Relocation entries are generated automatically by the assembler and automatically used by the link editor. Link editor options exist for both preserving and removing the relocation entries from object files.
The number of relocation entries for a section is found in the s_nreloc field of the section header. This field is a C language short and can overflow with large objects. If this field overflows, the section header s_flags field has the S_NRELOC_OVFL bit set. In this case, the true number of relocation entries is found in the r_vaddr field of the first relocation entry for that section. That relocation entry has a type of R_ABS, so it is ignored when the relocation takes place.